In recent years, magnetic recording apparatuses such as a magnetic disk apparatus, a flexible disk apparatus and a magnetic tape apparatus are widely used and their importance is increasing. Recording density of a magnetic recording medium used in the magnetic recording apparatuses is greatly enhanced. Especially, since the development of MR head and PRML technique, the areal recording density is more and more increasing. Recently GMR head and TuMR head have been developed, and the rate of increase in the plane recording density is very remarkable.
There is still increasing a demand for further enhancing the recording density in magnetic recording media, and therefore, a magnetic layer having a higher coercive force and a higher signal-to-noise (S/N) ratio, and a high resolution are eagerly desired.
In longitudinal magnetic recording media heretofore widely used, a self-demagnetization effect becomes significantly manifested, that is, adjacent magnetic domains in magnetic transition regions exhibit a function of counteracting the magnetization each other with an increase in a line recording density. To minimize the self-demagnetization effect, thickness of the magnetic recording layer must be reduced to enhance the shape magnetic anisotropy.
However, with a decrease in thickness of the magnetic recording layer, the magnitude of energy barrier for keeping the magnetic domains approximates to the magnitude of heat energy, and consequently, the heat fluctuation occurs, i.e., the recorded magnetization is reduced by the influence of the temperature. This undesirable phenomenon puts an upper limit on the line recordation density.
Recently, an anti-ferromagnetic coupling (AFC) medium has been proposed as means for solving the above-mentioned problem of limitation in the line magnetic recording density in the longitudinal magnetic recording media, which problem arises due to the alleviation of magnetization upon heating.
Perpendicular magnetic recording media attract widespread attention as means for enhancing the plane magnetic recording density. The perpendicular magnetic recording media are characterized in that the magnetization occurs in a direction perpendicular to the major surface of the magnetic recording media, which is in a contrast to the transitional longitudinal magnetic recording media wherein the magnetization occurs in an in-plane direction. Due to this characteristic, the undesirable self-demagnetization effect as encountered as an obstacle for enhancing the line recording density in the longitudinal magnetic recording media can be avoided, and the magnetic recording density can be more enhanced. Further, the thickness of magnetic recording layer can be maintained at a certain level, and thus, the problem of alleviation of magnetization upon heating as encountered in the traditional longitudinal magnetic recording media can be minimized.
In the manufacture of perpendicular magnetic recording media, a seed layer, an underlayer, a magnetic recording layer and an overcoat are usually formed in this order on a non-magnetic substrate. Further, a lubricating layer is often formed on the uppermost overcoat. In many recording media, a soft magnetic layer is formed under the underlayer. The underlayer is formed for the purpose of improving the characteristics of the magnetic recording layer, and the seed layer is formed for the purpose of providing desired crystal orientation and controlling the size of magnetic grains.
To produce perpendicular magnetic recording media having a high recording density and other improved magnetic characteristics, the crystalline structure of the magnetic recording layer is important. More specifically, in perpendicular magnetic recording media, the crystalline structure in the magnetic recording layer is often a hexagonal close-packed (hcp) structure. In this crystalline structure, it is important that the (002) crystal plane is parallel to the substrate surface, that is, the crystal c-axes ([002] axes) are aligned in the direction perpendicular to the substrate surface with minimized disturbance.
However, a perpendicular magnetic recording medium has a problem such that the total thickness of the magnetic recording medium is larger than that of the conventional longitudinal magnetic recording medium, although the perpendicular magnetic recording medium is advantageous in that the thickness of a magnetic recording layer can be relatively large. The formation of the perpendicular magnetic recording medium having a large thickness easily causes stacking faults in the crystalline structure at the steps of layers.
To align the crystal grains in the perpendicular magnetic recording medium without disturbance, an underlayer comprised of ruthenium having a hcp structure is usually formed in the medium, which is similar to the conventional longitudinal magnetic recording medium. In this medium, crystals in the magnetic recording layer epitaxially grow on (002) crystal plane of ruthenium, and thus, the magnetic recording medium exhibits enhanced crystalline orientation (see, for example, patent document 1).
The seed layer formed beneath the underlayer has a purpose of enhancing the crystalline orientation in the underlayer. Therefore, the seed layer has hitherto been formed from an amorphous material so as to have a smooth surface parallel to the substrate surface (see, for example, patent document 2).
The (002) crystal plane of the hop structure is orientated preferentially on (111) crystal plane of fcc structure, and therefore, the seed layer has also hitherto been formed from crystalline material having a fcc structure (see, for example, patent document 3).
In another aspect, the seed layer is required to be composed of fine crystal grains for giving a magnetic recording medium having improved recording/reproducing characteristic. However, in the case when the seed layer is comprised of an amorphous material, the crystal grains in the magnetic recording layer are not uniform in diameter and have large distribution of grain diameters. In the case when the seed layer is formed from a fcc crystal material, the size of crystal grains in the magnetic recording layer is difficult to control. Thus, the seed layer comprised of an amorphous material or a crystalline material with a fcc structure is difficult to give a magnetic recording medium having a satisfying recording and reproducing characteristics.
Thus, it is still eagerly desired to give a perpendicular magnetic recording medium having a seed layer comprised of a material having a uniform grain size distribution and capable of enhancing the crystalline orientation in the underlayer, and thus, which medium has improved recording/reproducing characteristic, and can be produced without difficulty.    Patent document 1: JP 2001-6158 A1    Patent document 2: JP 2004-70980 A1    Patent document 3: JP 2003-77122 A1